This application claims the priority of German patent documents 198 32 196.1, filed Jul. 17, 1998; 19855 775.2, filed Dec. 3, 1998; and 198 36 297.8, filed Aug. 11, 1998, the disclosures of which are expressly incorporated by reference herein.
The invention relates to a method for controlling the movement of an armature of an electromagnetic actuator, particularly for operating a charge cycle lifting valve of an internal-combustion engine, in which the armature oscillates between two solenoid coils against the force of at least one restoring spring, in response to alternating energizing of the solenoid coils. With an approach of the armature to the first-energized coil, during the so-called capturing operation, the electric voltage which is applied to the coil capturing the armature is reduced. An example of such a technical environment is disclosed in German Patent Document DE 195 30 121 A1.
A preferred use of an electromagnetic actuator of this type is in an electromagnetically operated valve gear of internal-combustion engines. That is, the charge cycle lifting valves of a reciprocating piston internal-combustion engine are operated by such actuators in the desired manner, being opened and closed in an oscillating fashion. In the case of such an electromagnetic valve gear, the lifting valves are moved individually or in groups by way of electromechanical control members (the so-called actuators), and the point in time for the opening and the closing of each lifting valve can be selected in an essentially completely free manner. As a result, the valve timing of the internal-combustion engine can be optimally adapted to the actual operating condition (defined by the rotational speed and the load) as well as to the respective demands with respect to consumption, torque, emissions, comfort and response behavior of a vehicle driven by the internal-combustion engine.
The essential components of a known actuator for operating the lifting valves of an internal-combustion engine include an armature, two solenoids (for holding the armature in the "lifting valve open" and the "lifting valve closed" position) with the pertaining solenoid coils, and restoring springs for the movement of the armature between the "lifting valve open" and "lifting valve closed" positions. For this purpose, reference is also made to the attached FIG. 1 which illustrates such an actuator with an assigned lifting valve in the two possible end positions of the lifting valve and of the actuator armature. Between the two illustrated conditions or positions of the actuator--lifting valve unit, the course of the armature lift z or of the armature path between the two solenoid coils, and the course of the current flux I in the two solenoid coils are illustrated over the time t corresponding to a known prior art (which is simpler than the initially mentioned German Patent Document DE 195 30 121 A1).
FIG. 1 shows the closing operation of an internal-combustion engine lifting valve 1 which moves in the direction of its valve seat 30. In a conventional manner, a valve closing spring 2a is applied to this lifting valve 1. The actuator, which as a whole has the reference number 4, acts upon the stem of the lifting valve 1--by means of a hydraulic valve compensating element 3 (which however, is not absolutely necessary). In addition to two solenoid coils 4a, 4b, this actuator 4 consists of a push rod 4c which acts upon the stem of the lifting valve 1 and carries an armature 4d which is guided to oscillate longitudinally between the solenoid coils 4a, 4b. A valve opening spring 2b is also applied to the end of the push rod 4c facing away from the stem of the lifting valve 1.
This is therefore an oscillatory system for which the valve closing spring 2a and the valve opening spring 2b form first and second restoring springs, for which therefore in the following the reference number 2a, 2b will also be used. The left-hand side of FIG. 1 shows the first end position of this oscillatory system, in which the lifting valve 1 is completely open and the armature 4d rests on the lower solenoid coil 4b (hereinafter also called an opener coil, since it holds the lifting valve 1 in its open position). The right-hand side of FIG. 1 shows the second end position of the oscillatory system in which the lifting valve 1 is completely closed and the armature 4d rests against the upper solenoid coil 4a (hereinafter also called a closer coil, since it holds the lifting valve 1 in its closed position).
In the following, the closing operation of the lifting valve 1 will be briefly described; that is, in FIG. 1, the transition from the left-side condition into the right-side condition. In between, the corresponding courses of the electric currents I flowing in the coils 4a, 4b as well as the lifting course or the path coordinate z of the armature 4d are each entered over the time t. With respect to the path constant z, the value z.sub.0 corresponds to a completely open lifting valve 1 (the armature 4d resting on the opener coil 4b), while in the case of z=z.sub.1, the armature 4d rests against the closer coil 4a.
Starting from the left-side "lifting valve open" position, the opener coil 4b is energized first in order to hold the armature 4d in this position against the tensioned valve closing spring 2a (=lower first restoring spring 2a), and the current I in the coil 4b is illustrated by a broken line in the I-t diagram. If the current I of the opener coil 4b is now switched off for a desired transition to "lifting valve closed", the armature 4d detaches from this coil 4b and the lifting valve 1 is accelerated by the tensioned valve closing spring 2a approximately to its center position (upwards). Because of its mass (moment of inertia) it continues to move, and in the process tensions the valve opening spring 2b so that the lifting valve 1 (and the armature 4d) are braked. Subsequently, the closer coil 4a is energized at a suitable point in time. (In the I-t diagram, the current I for the coil 4a is illustrated by a solid line.) In this manner, this coil 4a captures the armature 4d (the so-called capturing operation), and finally holds it in the "lifting valve closed" position illustrated on the right-hand side. After the armature 4d has been securely captured by the coil 4a, a switching takes place in the latter to a lower holding current level (compared I-t diagram).
The reverse transition from "lifting valve closed" to "lifting valve open" takes place analogously, from the position illustrated on the right-hand side in FIG. 1, by switching off the current I in the closer coil 4a and a time-shifted switching-on of the current for the opener coil 4b. In general, for energizing the coils 4a, 4b, a sufficient electric voltage is applied to them, while the switching-off of the electric current I is initiated by a reduction of the electric voltage to the "zero" value. The required electric energy for the operation of each actuator 4 is taken either from the electrical system of the vehicle driven by the pertaining internal-combustion engine or is provided by way of a separate energy supply adapted to the valve gear of the internal-combustion engine. In this case, the electric voltage is kept constant by means of the energy supply, and the coil current I of the actuators 4 assigned to the internal-combustion engine lifting valves 1 is controlled by a control apparatus, such that the required forces for the opening, closing and holding of the lifting valve or valves 1 in the respective desired position are obtained.
In the case of the above-explained state of the art, during the so-called capturing operation (in which one of the two coils 4a, 4b endeavors to capture the armature 4d), the coil current I is controlled by the above-mentioned control apparatus or by a control unit, by timing to a constant value which is high enough for securely capturing the armature 4d under all conditions. Now the force of the capturing solenoid coil 4a or 5b onto the armature 4d is approximately proportional to the current I and inversely proportional to the distance between the coil and the armature. If now - as in the known state of the art--a constant current I is adjusted, the magnetic force acting upon the armature 4d, with its approach to the respective coil 4a or 4b capturing it, rises inversely proportionally to the remaining gap, whereby the armature acceleration and armature speed rise. This results in a high impact speed of the armature 4d onto the respective solenoid coil 4a or 4b, which not only leads to high wear in the actuator 4, but also generates considerable noise. Another disadvantage is the switch-over loss of the transistors, which occur in the case of the briefly described switched current control and result in an increased power consumption and temperature-caused stress of the control apparatus, as well as in an increased electromagnetic radiation in the feeds of the actuators.
Improvements, particularly with respect to the generation of noise and wear of the actuator, are provided by the initially mentioned German Patent Document DE 195 30 121 A1 which discloses a method for reducing the impact speed of an armature onto an electromagnetic actuator. With an approach of the armature to the pole surface of the coil capturing the armature, the voltage applied to the coil is limited (that is, essentially reduced) to a definable maximal value so that the current flowing through the coil drops during a part of the time of the voltage limitation. The extent of the voltage limitation or voltage reduction can be defined in a characteristic diagram; thus, the corresponding values and particularly also the respective point in time at which this voltage reduction is to start must be determined experimentally.
It is an object of the present invention to provide further improvements with respect to the above; that is, to provide a simple and efficient method for reducing the impact velocity of an armature of an electromagnetic actuator.
This object is achieved by the method according to the invention, in which the capturing phase of the capturing operation is followed by a braking operation in which a switched electric voltage is applied to the coil until the armature impacts on the coil. The respective switching points in time and the voltage--switching ratio are determined by a controller by means of a desired trajectory describing the desired movement of the armature.
In a preferred embodiment of the invention, in addition to the voltage--switching ratio, the controller also determines the preceding sign of the voltage value whose amount is constant. That is, in a switched manner, either a positive or a negative voltage value or the "zero" voltage value is applied to the coil capturing the armature.
In general, it is suggested according to the present invention to replace the known current control or voltage reduction (which is to be determined empirically) during the capturing operation by a control which, during the so-called braking phase of the capturing operation, shortly before the armature impacts on the magnetic coil capturing it, applies electric voltage to this coil in a controlled manner, (specifically in a switched manner). The respective switching points in time for the switching-on and switching-off of the electric voltage (as well as optionally also their preceding signs) are determined by means of a desired trajectory describing the desired movement of the armature.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.